Contact between tubes and heat exchanging fins in heat exchangers/radiators installed in cars is today ensured mainly by two methods--mechanical expansion of tubes, or brazing of tubes to heat exchanging fins. The reliable contact achieved by the mechanical expansion of the tubes can be achieved only when using substantially round tubes, which means limited heat performance of the heat exchangers. Brazed flat copper or brass tubes are largely used in car radiators, providing a good performance when the inexpensive well-known brazing technique is used.
There is an increasing trend in the automotive industry to replace steel and copper with lighter materials like aluminum or magnesium.
Soldering and brazing of aluminum to aluminum or to other metals is, however, a difficult task because of the presence of an aluminum oxide layer formed instantly on the surface of aluminum members exposed to the atmosphere. This oxide layer prevents formation of a tight metallic bond between brazed members because of its poor wetting properties.
In practice it is therefore necessary to pretreat the aluminum surface prior to soldering/brazing in order to remove the aluminum oxide. Different methods are used for removing the oxide layer, depending on the subsequent use of the aluminum members. A method called flux brazing is widely used where the aluminum members are immersed in a bath of molten salt and brazing flux or exposed to the brazing flux which removes the oxide layer from the preheated members. The method is quite efficient with regard to the oxide removal, but it is practically impossible to avoid that the flux is entrapped at the brazed joints representing a serious corrosion problem.
Moreover, the brazing material commonly used is an Al-Si alloy which is a high temperature brazing alloy. In practice when the units to be brazed are assembled with the parts in position on the coated tubes, the assembly is then placed in a vacuum furnace and heated. However, the melting temperature of this brazing alloy is around 1050.degree. to 1100.degree. F. which is not far below the melting point of aluminum, and as a result the temperature must be very closely controlled. But the silicon from the brazing alloy vaporizes and tends to deposit on surfaces within the heating apparatus, including the temperature sensors, thereby making control uncertain at best. Thus the process is not very reliable or efficient.
Fluxless brazing processes have been conducted successfully in vacuum or inert atmosphere and are described in U.S. Pat. Nos. 3,373,482 and 3,979,042.
The common drawback of these processes is that the necessity of cleaning/pretreating procedures is not eliminated and rather high temperatures are required in order to break down the oxide layer or to create cracking in this layer. These processes are therefore expensive and limited to special applications.